An optical fiber cable is typically constructed of a glass or plastic core. The core is generally configured in a cylindrical shape and extends along the length of the cable. The core is encased within a cladding material constructed of a glass material which also extends along the length of the cable. The cladding material is constructed of a glass or plastic material which is different in composition from the material from which the core is constructed. The cladding material will typically have a lower refractive index than the refractive index of the material which is used to construct the core. In various constructions of optical fiber cable, the cladding material is surrounded by a buffer material such as a protective coating or a protective encasement constructed of high strength fibers. The optical fiber cable will often also include an outer protective jacket constructed of a strong durable material which surrounds the buffer material.
An optical fiber cable is used in different applications and within different environments. The core will carry a transmitted light beam which, in many examples, will carry data. The light beam signal transmits data within the core at a high rate of speed and the core provides a broader bandwidth than more traditional metallic cable.
In some constructions of an optical fiber cable, multiple cores are provided which are positioned within the cladding material of the cable. These multicore optical cables are similarly constructed as the single core optical fiber cable, however, the multiple cores in the multicore optical fiber cable are positioned within the cladding material spaced apart from one another at known distances. In a normal operating condition with the cable extending in a relatively straight orientation which may include relatively gentle curvatures, the cores are isolated from one another from cross communication between cores.
The transmission of a light beam in a first core of the multiple cores, generates an evanescent field that extends beyond the boundary surface of the first core. The evanescent field extends into the lower refractive indexed cladding material. With a second core present having a higher refractive index than that of the cladding material and with the second core positioned in close enough proximity to the first core, a phenomenon of evanescent coupling takes place between the first and second cores. As mentioned above, the separation between cores within a multiple core optical fiber cable will typically position, for example, a first and second core sufficiently far enough apart such that a signal transmitted within the first core will not affect the second core. However with a first and second core moved to a position such that they are in close enough proximity to one another, the evanescent field created by the transmission within the first core will affect and influence the second core. Bending of a core carrying a signal also causes the shape of the evanescent field to change shape. If the core is bent past a critical point the field may interact with an adjacent core. These affects are commonly known as an evanescent coupling phenomenon which results in a transfer of energy to the second core.
Should the second core not be transmitting a signal at the time the evanescent field becomes present, the evanescent coupling will propagate a signal in the second core. This propagated signal in the second core can be detected and measured. Should the second core already be transmitting a signal at the time of the evanescent coupling, the evanescent coupling will alter that signal within that second core. The altered signal in the second core can also be detected and measured.
When a light beam is transmitted in a first core of a multiple core optical fiber cable, where the cable is positioned to extend in a relatively straight orientation or with gentle curves, the light beam internally reflects at the boundary of the first core as the light beam is transmitted along the core without cross communication occurring between cores in the multicore optical fiber cable. However, should the multicore optical fiber cable be positioned with a sufficient bend in the cable, the angle of incident of the light beam on the boundary of the first core carrying a light beam transmission will change as a result of the bend in that core in which the light beam is transmitted. Should the angle of incident of the light beam transmission exceed a critical angle for the material from which the first core is constructed, at least a portion of the light beam will be refracted and be transmitted into the cladding material. This transmission of the light beam signal can interact with a nearby second core and result in the refracted light beam being transmitted along a second core. The transmission of this transmitted light beam within the second core can also be detected and measured.
With a light beam signal being transmitted in a first core, the transfer of energy to a second core can be facilitated through the operation of the evanescent coupling and/or by way of the light refraction phenomena. Thus, with moving a first and a second core within a distance of the field of influence of the evanescent field and changing the distance between the first and second core within the field of influence will result in changing the amount of energy being transferred to the second core. As the first and second core move closer together the transfer of energy will be greater to the second core and as the distance of separation increases the amount of energy transferred to the second core is diminished. In the instance of bending of the cable, this results in the bending of the boundary of the first core. Once the internally reflected light beam within the first core exceeds beyond the critical angle of the first core, the amount of energy refracted to and transmitted to the second core increases and correspondingly the amount of energy transmitted decreases as the bend returns the angle of incident of the light beam closer to the critical angle. Thus, with a change of the optical fiber cable configuration along with a transmission of a light beam along a first core, a detectable and measurable energy transfer into the second core as a result of one or both of the phenomena will take place.
With the cores within a multicore optical cable being constructed of a material such as glass, the glass is a substantially inert material, having low thermal expansion coefficients and is resistant to compressive strain. Thus, in order to carry out induced evanescent coupling and/or transmission of light beam energy from a first to a second core within a multicore optical fiber cable, localized alteration of the shape of the cable needs to take place which can effect distance between the first and second cores within the optical fiber cable and/or localized alteration of the shape of the cable with bending of the optical fiber cable which results in bending of the cores within the cable needs to be accomplished. This localized manipulation of the optical fiber cable needs to be usable and flight worthy reliable. Such manipulation can result in the use of multicore optical fiber cable, with appropriate calibration as needed, as a sensor, switch or modulator.
The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings.